1879. 1 on the ' Thunderer ' Gun Explosion. 



To cause the shells to obey the action of the rifling, and to rotate 

 on their axes, they are provided with as many rows of studs as there are 

 rifle grooves — namely, nine rows, with three studs in each row. The 

 studs are secured into the shells in the following manner. Diagram 9 

 shows, greatly exaggerated by Fig. 1, a section through a stud-hole. 

 This is circular, and, as you will see, is undercut ; it is made by drilling 

 out a parallel hole, as represented by the dotted lines, and then by 

 introducing a tool, a sample of which I now show you, provided with 

 a hinged cutter, which i^rojects further and further as the tool 

 descends, until by the time it has reached the bottom of the hole the 

 projection is sufficient to give the undercut form shown. Fig. 2 repre- 

 sents one of the gun-metal studs placed in such a hole. You will see 

 the stud is cylindrical, but that the bottom of it is cupped. If severe 

 pressure be applied to the top of the stud, the cavity of the cup will 

 be flattened, and the inverted brim, so to speak, of the cup, will be 

 swelled outwards, and will fill up the countersunk part of the hole 

 (see Fig. 3), and in that way it will be securely fixed into the shell, 

 without the possibility of becoming unscrewed or detached. 



The Palliser chilled shell is too hard to admit of the stud-holes 

 being drilled out ; they are therefore formed in the act of casting, by 

 a process which is technically known as coring — that is to say, sand 

 is rammed into a box the shape of the desired hole ; the sand is then 

 dried, and becomes a plug which is fixed in the side of the chill mould ; 

 the metal flows round about the plug, and when the metal is set the plug 

 is cleared away, and a hole of the form of the box in which the sand 

 plug itself was made is left in the casting ; but such a hole will not have 

 the smooth surface of one that has been cut out by a drill. You 

 have before you samples of the cores, samples of the cored holes, and 

 samples of the drilled holes; also studs which have not yet been 

 compressed into a hole, and a number of studs which have been com- 

 pressed, some into drilled holes and some into cored holes. The 

 appearance of the parts which have been in these respective classes of 

 holes differs, as might be expected, bearing in mind that the one class 

 has been cut out so as to be smooth, while the other has been cored 

 out, and is comparatively rough. The difference is sufficiently marked 

 to enable one to say which of the studs have been in cored holes and 

 which of them have been in drilled holes. 



The projectiles do not fit the bore of the gun accurately, and thus 

 there is a space (a very small one, it is true, but still a space) 

 through which the gases from the ignited powder can pass between 

 the projectile and the walls of the bore. This escape of gas causes 

 a slight decrease in the useful effect of the powder, but is more preju- 

 dicial for another reason, namely, that the high velocity at which 

 the heated gases pass operates injuriously upon the bore of the gun 

 by erosion in the neighbourhood of the base of the projectile. To 

 prevent this escape and the injury arising from it, the base of the 

 projectile is provided with what is known as a " gas-check." Eevert- 

 ting to Fig. 9, you will see attached to the base of the shell a slightly 



Vol. IX. (No. 71.) r 



